1. Field of the Invention
The present invention relates to the field of fiber optics, and in particular to a method and an apparatus for a tunable holographic drop filter with a quasi phase-conjugate fiber coupling.
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2. Background Art
Digital and analog information is often communicated using optical fibers. In some schemes, many signals, each with its own optical wavelength, are communicated on the same optical fiber. At some point, it is necessary to extract a signal (i.e. a particular optical wavelength) from the optical fiber, and this is accomplished with a drop filter. A problem with prior art drop filters is that they are limited to a single fixed optical wavelength. This problem can be understood by a review of optical signal transmission schemes.
Optical Signal Transmission Scheme
With the increase of data transfer due to the popularity and ease of use of the Internet, there is a need to increase the volume (commonly termed bandwidth) of data that can be transmitted across a network of computing devices. Initially, optical fiber networks carried only a single signal at a single wavelength. A scheme using wavelength division multiplexing (WDM) has significantly enabled increases to the aggregate volume of data that can be transmitted over a network like the Internet.
The basic concept of WDM is to insert and remove multiple data channels in and out of an optical fiber. Prior to the use of WDM, most optical fibers were used to unidirectionally carry only a single data channel at one wavelength. WDM divides a network's bandwidth into channels, with each channel assigned a particular wavelength. This allows multiple channels (each at a different wavelength) to be carried on the same transmission medium simultaneously. Each data channel is transmitted at a unique wavelength, and the wavelengths are appropriately selected such that the channels do not interfere with each other, and the optical transmission losses of the fiber are low. The gain in the network bandwidth is given by the aggregation of multiple single channel bandwidths.
The channels in a WDM system are multiplexed at a transmitting end and transmitted to a receiving end where they are demultiplexed into individual channels. In the existing systems, the transmitting and receiving ends must be tuned to the same wavelengths to be able to communicate. That is, the transmitting and receiving ends use a device such as an add/drop multiplexer to transmit/receive a fixed wavelength channel.
In the case of fiber optic cable, optical add/drop multiplexers are used at the transmitting and receiving ends to combine and separate multiple channels. The transmitting end combines all the channels into a single fiber using a multiplexer. At the receiving end the channels are separated with a demultiplexer. However, if not all of the channels carried by the fiber are required at the receiving end (or an intermediate node), a single drop filter can be used in place of a demultiplexer to extract an individual channel. Existing systems can have as many as 160 signal channels from which to choose.
A limitation in current drop filter implementations is their fixed wavelength response (i.e. the drop filter is configured to extract only a specific wavelength from the optical fiber).